Downstream Migration of the European Eel (Anguilla Anguilla) in the Elbe River, Germany: Movement Patterns and the Potential Impact of Environmental Factors

Abstract

Recruitment of European eels (Anguilla anguilla) has declined to the extent that they have been added to the IUCN Red List of Threatened Species. Therefore, it is critical to ensure that eels complete their outward river migration in order to contribute to the available spawning stock. We conducted a 4-year (2007)(2008)(2009)(2010)(2011) telemetry study to understand the migratory behaviour and potential impact of environmental factors on the eel during this critical life stage.Out of 399 female eels tagged with acoustic transmitters, only 28% demonstrated clear downstream migratory behaviour. Fifty-five percent were detected exhibiting no downstream migration behaviour and 17% were not detected at any monitoring station. Movement patterns of downstream-migrating (silver) eels were characterized by nocturnal activity and seasonal migration, with distinct peaks in autumn and spring. Migration was often discontinuous and exhibited phases of active locomotion and expanded stopovers. The most important determinants of movement activity were water temperature, cumulative precipitation and moonlight, although the significance varied by season and location in the river basin.Our results evidence a discontinuous, stepwise migration over an extended period. Furthermore, our findings indicate that migration success depends on holding duration prior to tagging and environmental predictors with varying importance depending on the season, as well as the locations of capture, tagging and release.

Freshwater and coastal migration patterns in the silver‐stage eel Anguilla anguilla

“…A possible interpretation of migration patterns shown here, which has been raised by other studies, is that sea migration could be a two‐step migration process (Durif et al , ; Aarestrup et al , ; Béguer‐Pon et al , ; Stein et al , ). It has been reported that A .…”

“…Crook et al () demonstrated extended estuarine residence time for Anguilla australis Richardson 1841, highlighting the possibility of more complex migration behaviour instead of the rapid and direct seaward migration originally assumed. Stein et al () also highlighted the possibility that A . anguilla may need more than one migratory season to reach the sea and may temporally revert to a non‐migratory stage.…”

Behaviour, escapement and mortality of female European silver eels within a regulated lowland river draining into the Baltic Sea

“…The observed surgical infection rate (8%) and the associated mortality (2.65%) were in the range of reported values for the method used (Baras & Jeandrain, ). In general, implantation of telemetry transmitters has been shown to have little effect on the survival rate of eels (Simon, Berends, Dorner, Jepsen & Fladung, ; Stein et al., , Winter, Jansen, Adam & Schwevers, ), but possible surgically induced death after recovery and release could not be excluded (Stein et al., ). An activity‐reducing effect within the first 11 weeks after tagging and release has also been demonstrated (Winter et al., ).…”

“…Stein et al. () observed that the animals had to be kept for more than 20 days to terminate the migration of all the eels tested and that the rate of continuation of migration was highest after caging for up to 10 days. The reversal rate in the Warnow River after the first release was 21%, and 11% after recapture and re‐release.…”

“…In this context, the higher losses in the Unterwarnow compared with the Oberwarnow are likely connected with a higher presence of anglers, fishermen and animal predators (cormorants, Davidsen et al., ) in the estuary. In general, the fishing mortality rates reported in the literature fluctuate depending on the respective study area (range 8‐82 %, Aarestrup et al., ; Jansen, Winter, Bruijs & Polman, ; Simon et al., ; Stein et al., ; Winter, Jansen & Breukelaar, ). The documented fishing mortality for the Warnow River and the Baltic to Fehmarn was 6.8% and thus slightly below the figures for other areas.…”